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In 1972, the U.S. Air Force issued the Advanced
STOL transport program for a new medium airlifter that capable of
operations into battle zones with short, temporary airfields. Two
proposals were accepted for construction as the Boeing YC-14 and McDonnell
Douglas YC-15 prototypes. McDonnell Douglas's design incorporated a
supercritical wing, the result of NASA research. The design team also
chose to use externally double-slotted flaps to direct part of the jet
exhaust downwards, while the rest of the exhaust passed through the flap
and then followed the downward curve due to the Coandă effect. Two YC-15s
were built, the first flight was on 26 August 1975. The second prototype
followed in December. The Air Force decided it needed a larger transport
that would fly to standard, conventional airfields rather than into battle
zones. In 1979, the Air Force formally cancelled the AMST program for both
the YC-14 and the YC-15. As a result, the C-X program, the C-17 evolved.
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